Gravure printing is a method of printing which uses an etched or engraved cylinder. Ink occupies the depressions in the cylinder and is transferred to a print medium. Surface defects on the cylinder, such as pits or spots which are too hard or too soft result in engraving errors and subsequent need for repolishing and replating which is expensive and time consuming.
Since the development of automatic method of electronic engraving, the electrodeposition of copper of known physical and mechanical properties with reproduceable grain size, crystal structure and hardness over the entire surface of the cylinder is desirable. The copper plating processes, typically directed towards decorative plating, have as their objective to impart leveling and brightness characteristics with little regard to precise physical properties that are important for electronic engraving. Such decorative applications are generally concerned with deposits ranging in thickness from about 0.0005 to about 0.0015 inch while gravure rolls require deposits ranging from 10 to 20 times these thickness values.
For successful electronic engraving, the copper deposits must have reproducible grain size, crystal structure and hardness. One problem associated with copper deposits involves annealing. Annealing is a tendency of the hardness of the copper deposit to decrease with time as a result of changes in crystalline size, texture, microdeformations and dislocations within the copper deposit.
Certain acid copper plating baths are also known to perform differently with respect to the immersion depth of the rotating cylinder. The principal problem in this regard is annealing. This problem of recrystallization (annealing) is characteristic of totally submerged cylinder operations when using a bath designed for partial immersion such as described by U.S. Pat. No. 4,334,966. The same holds true of partially submerged cylinder operations when using a bath designed for total immersion such as described by U.S. Pat. No. 4,781,801.
It has been discovered that by incorporating an alkoxylated compound such as alkoxylated 2-mercaptoethanol or 2,2'-thiodiethanol into the acid copper baths the problem of annealing can be eliminated at any level of immersion.